1. Field of the Invention
This invention relates generally to antimicrobial peptides and more specifically to cryptdin peptides, nucleic acid molecules encoding cryptdins, and their uses.
2. Background Information
Survival in a world teaming with microorganisms depends on a network of host defense mechanisms. Among these mechanisms are phagocytosis by cells are resident in tissues or that circulate in the blood system and ingest, kill and digest potentially harmful microbes. Although pathogenic microbes may vary considerably, phagocytes are able to destroy the vast majority by sequestering them in intracytoplasmic vacuoles and exposing them to a lethal mixture of organic and inorganic toxins.
Perhaps the most remarkable ultrastructural feature of phagocytes are their several thousand cytoplasmic granules, which are membrane-bound organelles typically about 0.3 .mu.m in diameter. During phagocytosis, some of these granules fuse to phagocytic vesicles thus enabling the contents of the granule to enter the lumen of the vesicle. Early observers surmised correctly that the granules contained factors which were responsible for intraphagosomal killing in digestion of microbes. These granules contain a mixture of antimicrobial molecules including various peptides such as the so-called defensins.
Defensins are abundant antimicrobial peptide components of vertebrate neutrophil and macrophage granules. Members of the defensin family have been identified previously in human, rabbit, guinea pig and rat phagocytes, primarily those phagocytes termed phagocytic granulocytes. Defensins are cationic peptides that have molecular weights between about 3 and 4 kiloDaltons (kDa) and that exhibit broad-range antimicrobial activities against gram negative and gram positive bacteria, many fungi and some enveloped viruses. The peptides are characterized by eight invariant amino acids, including six invariant cysteine residues that constitute a unique disulfide motif. The three disulfide bonds stabilize a tertiary conformation consisting predominantly of .beta.-sheet. The highly ordered structure and the absence of a helix make defensins unique among known antimicrobial peptides. It appears that defensins exert their antibacterial effect by permeabilizing the cytoplasmic membrane of the target microorganism by a mechanism that may involve the formation of ion channels or transmembrane pores.
Until recently, defensins had been identified only in circulating or tissue phagocytes of myeloid origin. However, based on the presence of a particular mRNA, it has been surmised that similar peptides might be present in the epithelial cells of the small intestine. Such intestinal peptides may prevent access of microorganisms through the small intestine into the systemic circulation and, therefore, can be useful as a therapeutic or prophylactic agent. Thus, a need exists to identify peptides that have antimicrobial activity within the mucosal epithelium or in the intestinal lumen. The present invention satisfies this need and provides additional benefits as well.